1. Field of the Invention
The present invention relates to disk drives for computer systems. More particularly, the present invention relates to a disk drive employing seek time VCM IR voltage calibration for velocity control of an actuator arm.
2. Description of the Prior Art
Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and embedded servo sectors. The embedded servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo control system to control the velocity of the actuator arm as it seeks from track to track.
There are times when the servo control system does not have access to the embedded servo sectors yet it is still desirable to control the velocity of the actuator arm. For example, in disk drives wherein the head is parked on a landing-zone of the disk, it is desirable to control the velocity of the actuator arm to unlatch the head during spin-up. In disk drives employing ramp loading/unloading, it is desirable to control the velocity of the actuator arm so that the head is not damaged as it travels off the ramp onto the disk as well as off the disk onto the ramp. Another example is if the servo control system loses servo sector synchronization it is desirable to control the velocity of the actuator arm to facilitate re-synchronizing to the servo sectors.
Prior art techniques for controlling the velocity of the actuator arm when servo sector information is unavailable include using a voltage loop with the detected back EMF generated by the VCM as the feedback. The VCM is essentially an RLC circuit where R is resistance, L inductance, and C the inertia of the motor and load. The voltage contribution of C to the measured back EMF is proportional to the velocity of the VCM. Since the resistance R is in series with C, it is desirable to cancel R""s contribution to the back EMF leaving only LC. Once the resistance R is canceled, at low frequencies Ldi/dt is small leaving the voltage contribution of C as the dominant factor in the measured back EMF.
Prior art techniques for performing VCM resistance compensation include calibrating and subtracting from the measured back EMF the voltage contribution of R (i.e., the IR voltage where I is the current in the VCM). The VCM resistance R is measured by applying a fixed current to the VCM in order to press the actuator arm against a fixed object (e.g., the crash-stop for stopping the head at the inner diameter (ID)). With the actuator arm pressed against the fixed object, the velocity is zero and Ldi/dt is zero, leaving the VCM resistance R as the only contribution to the measured back EMF. However, the calibrated R value is only valid for a few seconds since the VCM resistance R can fluctuate significantly with temperature. If the VCM cools down, the IR voltage can become negative resulting in instability and damage. If the VCM heats up, the IR voltage can mask the velocity measurement (the voltage contribution of C), resulting in a failure to load or unload. Thus, it would be necessary to move the actuator arm to the fixed object every few seconds to perform the above calibration procedure to update the estimate for the VCM resistance R. This problem can be alleviated by using higher performance VCMs (e.g., VCMs with a higher Km); however, this increases the cost of the disk drive.
There is, therefore, a need to reduce the cost of a disk drive by employing a less expensive VCM, and to improve performance by calibrating the VCM IR voltage to enable velocity control of an actuator arm without having to periodically move the actuator arm to a fixed object.
The present invention may be regarded as a disk drive comprising a disk, a head, an actuator arm for actuating the head radially over the disk during a seek operation, and a voice coil motor (VCM) for rotating the actuator arm about a pivot, the VCM comprising a coil comprising a VCM resistance R. A back EMF voltage detector measures a back EMF voltage across the coil, and a current detector detects a current I flowing through the coil. An IR voltage detector, responsive to the current I detected by the current detector, detects an IR voltage proportional to the current I times the VCM resistance R. A voltage compensator substantially cancels the IR voltage from the measured back EMF voltage to generate a compensated back EMF voltage. A control voltage generator, responsive the compensated back EMF voltage, generates a control voltage applied to the coil to generate the current I flowing through the coil. A memory stores a first compensated back EMF voltage generated at a first time period of the seek operation. A comparator compares the first compensated back EMF voltage stored in the memory to a second compensated back EMF voltage generated at a second time period of the seek operation. A calibrator, responsive to the comparator, calibrates the IR voltage detector.
In one embodiment, the disk drive further comprises a current integrator for integrating the current I detected by the current detector between the first and second time periods, wherein the second time period occurs when the integrated current I crosses zero. In another embodiment, the memory comprises a capacitor. In an alternative embodiment, the memory comprises an analog-to-digital converter and a register. In yet another embodiment, the comparator comprises an analog-to-digital converter.
The present invention may also be regarded as a method of controlling velocity of an actuator arm in a disk drive. The disk drive comprises a disk, a head, the actuator arm, and a voice coil motor (VCM) for rotating the actuator arm about a pivot. The VCM comprises a coil comprising a VCM resistance R. A control voltage is generated from a command input and a compensated back EMF voltage. The control voltage is applied to the coil to generate a current I flowing through the coil to move the actuator arm during a seek operation. A back EMF voltage across the coil, a current I flowing through the coil, and an IR voltage proportional to the current I times the VCM resistance R, are detected. The IR voltage is subtracted from the back EMF voltage detected across the coil to generate the compensated back EMF voltage. During a first time period of the seek operation, a first compensated back EMF voltage is generated and stored in a memory. During a second time period of the seek operation, a second compensated back EMF voltage is generated. The first compensated back EMF voltage stored in the memory is compared to the second back EMF voltage, and the result of the comparison is used to calibrate the detected IR voltage.